1. Field of the Invention
This invention relates to the synthesis of ferromagnetic chromium dioxide. More particularly, this invention provides a two-stage, partitioned reduction process for synthesis of ferromagnetic chromium dioxide with certain organic reducing agents.
2. State of the Art
Ferromagnetic chromium dioxide is generally manufactured in a two-stage process. In the first stage, chromium trioxide (CrO.sub.3) is dissolved in water under ambient conditions, and chromic oxide (Cr.sub.2 O.sub.3) is then added to form a paste. Dopants and crystal modifiers, such as iron and antimony, may be added to achieve desired magnetic properties. In the second stage, this highly viscous paste is transferred to a reaction vessel, such as an annular can as disclosed in U.S. Pat. No. 4,054,544, where the paste is heated to an elevated temperature under pressure (e.g. 350.degree. C. under 340 atmospheres) to prepare acicular crystals of chromium dioxide.
While this process can produce chromium dioxide with an excellent range of magnetic properties, the process is not optimum in several respects. Paste produced in the first stage contains insoluble chromic oxide particles that serve as nucleation sites for CrO.sub.2 crystal growth during the second stage. The CrO.sub.2 crystals that form on these insoluble particles are relatively large compared to those derived from crystal growth modifiers such as Sb.sub.2 O.sub.3. The net effect is a relatively broad CrO.sub.2 crystal size distribution that is undesirable for many applications.
Secondly, paste in the first stage tends to become so viscous that it is difficult to handle. The mixing step must be abbreviated to permit transfer from the mixing vessel to the reaction vessel before the paste becomes too viscous to pour. Adding more water to dilute the paste, and thereby reduce the viscosity, is not desirable since dilution adversely affects quality of derived CrO.sub.2 and would have a negative effect on productivity and production capacity.
Thirdly, heat is added to the paste in a closed vessel during the second stage, to raise it to temperatures in the vicinity of 350.degree. C. where the paste is converted to ferromagnetic chromium dioxide in a series of complex reactions. Because of its bulk, heating of the unstirred paste to reaction temperature is not only slow, but also creates thermal gradients within the paste. Differing thermal history throughout the reaction mass adversely affects uniformity of the chromium dioxide, and thus its magnetic properties.
Therefore, there is a need for an improved process for the synthesis of ferromagnetic chromium dioxide.